1. Field of the Invention
The invention pertains to apparatus and methods for detecting neutrons and more particularly to neutron detectors having an ionizable gas contained in a dielectric capsule.
2. Description of Related Art
Kocsis has published several papers describing a particle detector using a syntactic foam comprising gas-filled glass micro bubbles embedded in epoxy and held in an external electric field [see, e.g., M. Kocsis, The micro void neutron detector, Nucl. Instr. and Meth. in Physics Res. A, 529, 354-7, 2004]. In operation the applied electric field is held below the dielectric breakdown threshold. When ionizing radiation is present, free electrons are liberated within the gas volume and are multiplied by further collisions according to well-understood physical principles. As the charges reach the opposite surfaces of the void, an induced electric pulse is detected by the external electrodes. Kocsis notes that the detector may be sensitive to various types of ionizing radiation such as X-rays and that sensitivity to neutrons may be enhanced by filling the bubbles with 3He.
The detector taught by Kocsis was shown to detect radiation from a 90Sr source as well as cold neutrons. However, it has several shortcomings that limit its usefulness for many applications: First, the size of the micro bubbles was too small to capture all the energy of the ionizing particle, as stated on p. 356 of the aforementioned paper: “There is no full energy deposition in the void and the signal is generated mainly by the void discharge . . . . Therefore the amplitude spectrum is similar for different ionizing particles. The main difference is in the efficiency of triggering the discharge in the voids due to different specific ionization of the particles.” In other words, the detector is operated substantially in the Geiger-Mueller mode. A second shortcoming is that, although Kocsis suggests using 3He as the void gas, the size and composition of the glass micro bubbles (1.5 μm wall thickness, borosilicate glass) would be generally unsuitable for containing He, because it would lose roughly 50% of the 3He pressure within three days. A third shortcoming is that the large number of layers of micro bubbles between one pair of electrodes limits the output signal.
Henderson et al. disclose neutron detectors based on 3He-filled glass shells embedded in a matrix of scintillating material with event detection using a photomultiplier tube (PMT). Two publications appeared in the scientific literature [G. F. Knoll, T. M. Henderson, W. J. Felmlee, “A Novel 3He Scintillation Detector,” IEEE Transactions on Nuclear Science NS-34 [1], pp. 470-474 (1987); G. F. Knoll, T. F. Knoll, T. M. Henderson, “Light Collection in Scintillation Detector Composites for Neutron Detection,” IEEE Transactions on Nuclear Science 35 [1], pp. 872-875 (1988)] and a patent was granted for this approach [Henderson et al., U.S. Pat. No. 4,795,910, “Radiation-detection/scintillator composite and method of manufacture”]. The Henderson device was capable of improved 3He gas retention as compared to the Kocsis device, but would still lose the majority of its 3He gas fill in less than a year, a problem for long-term (practical) use in the field. Henderson et al make a number of suggestions for improving the performance of their device; however, their suggestions relate to their particular detection methodology.
Lorikyan has disclosed the use of a porous dielectric material for alpha, beta (electron), and neutron detection [“The porous multiwire detector,” M. Lorikyan, Nuclear Instruments and Methods in Physics Research A, 454, Issue 1, 257-59 (2000)]. In this device, alphas and betas interacting in the device yield electrons in the walls of the porous material that then travel through the pores, striking the walls of the pores as they go, thereby yielding secondary electrons and producing gas gain.
A number of patents granted to George et al. and others at SAIC disclose a light-emitting panel and various components and manufacturing methods connected therewith [see, for example, U.S. Pat. Nos. 6,545,422; 6,570,335; 6,612,889; 6,620,012; and 7,125,305]. Although these patents are directed primarily to light-emitting (flat-panel) displays, the disclosures mention the possibility of using them for particle detection: “Additionally, the light-emitting panel may be used for particle/photon detection. In this embodiment, the light-emitting panel is subjected to a potential that is just slightly below the write voltage required for ionization. When the device is subjected to outside energy at a specific position or location in the panel, that additional energy causes the plasma forming gas in the specific area to ionize, thereby providing a means of detecting outside energy.” [Johnson et al., '012 Col. 4 lines 33-40]. Because these patents are directed to light-emitting panels, it appears that Johnson et al. contemplate detecting a particle event through similar optical emission, but it is not stated whether this is the case or whether it is contemplated to detect the event by sensing current flow between adjacent electrodes. In any case, these patents contemplate operating the panel at “a potential that is just slightly below the write voltage required for ionization.” This is equivalent to operation in the Geiger-Mueller mode, where the pulse amplitude is independent of particle energy and thus does not provide information on the type or energy of the incoming particle.
Drukier et al have described a neutron detector based on superconducting grains embedded in a dielectric matrix material. In essence, neutron energy deposition in a superconducting grain heats the grain and changes its properties. [Drukier, A. K., Igalson, J., and Sniadower, L., “A new detector of neutrons,” Nuclear Instruments and Methods, 154 [1], pp. 91-4 (1978)]. Superconducting grains are set, read out, and reset using an externally applied magnetic field. Neutron-gamma discrimination is done on the basis of a certain minimum energy deposition in a superconducting grain being necessary to change the state of the superconducting grain. Because of this characteristic, the Drukier device does not yield a direct electronic pulse that can be evaluated using pulse height analysis (pulse height discrimination) for neutron-gamma separation.
Tomassino et al. in U.S. Pat. No. 4,330,710 describe a neutron dosimeter whose body is formed from a transparent dielectric material and in which a second material is contained inside, with an optical readout method being used. The Tomassino device does not provide real-time detection (i.e., it measures integrated neutron exposure and does not indicate individual neutrons).
McGregor et al in US Patent Application 2006/0043308 disclose an array of micro neutron detectors based on components (usually two substrates with cavities in them) that are fit together in a gaseous environment to form a gas-filled pocket that acts as a neutron detector. Neutron sensitivity is achieved by having a neutron reactive material (in other words, neutron target material) present in the detector, such as a thin layer (e.g. 10B, 6Li, 235U) coating one of the interior surfaces of the detector. As is described in the disclosure, when assembled together, a detector consists of an outer body of insulating material with electrode wires entering the detector through holes in the insulating outer body and connecting to electrodes inside the outer body; in other words, electrodes are internal, rather than external. McGregor et al disclose additional variations and uses of this concept in US Patent Applications 2006/0023828 and 2006/0056573.